1. Field of the Invention
This invention relates generally to the use of microelectromechanical or micromachined devices manufactured by semiconductor processing techniques for the measurement of stress or pressure, and more particularly to the use of magnetoresistive sensors in conjunction with microelectromechanical devices for use as stress/pressure sensors, temperature sensors and magnetic field sensors, including an improved fingerprint sensing device.
2. Description of the Related Art
There are two well known and published types of semiconductor based sensors for pressure sensing. The first is a pressure sensor as described in U.S. Pat. Nos. 5,316,619 and 5,888,845, which describe the use of a capacitor for measuring pressures within sealed chambers. As shown in FIG. 1, the sealed chamber 104 of capacitive sensor 100 has a top defined by a very thin diaphragm 106 of known and fixed length and thickness (length typically in the range of 200 microns and thickness typically in the range of 3–4 microns); a bottom wall (not shown) that is at a fixed predetermined distance from the diaphragm (typically 0.2 micron away), which is essentially composed of the silicon substrate 102, and side walls 108 that support the top membrane. The diaphragm and the bottom of the chamber form the two plates of a parallel plate capacitor. Upon the application of pressure to the diaphragm, it deflects towards the bottom of the chamber, and the reduced spacing between the diaphragm and the bottom of the sealed chamber results in a change in capacitance.
U.S. Pat. No. 5,316,619 teaches some design rules for the dimensions of the diaphragm in order to achieve a certain minimum nominal capacitance, and also to obtain a minimum sensitivity, so as to make the capacitive sensor viable for use as a product. Using these design rules, the required dimensions of the diaphragm are of the order of 500 microns in length and 4 microns in thickness. U.S. Pat. No. 5,888,845 uses similar design concepts and capacitive measurement techniques, but teaches an improved manufacturing process to obtain diaphragms that are substantially thinner, on the order of 0.02–1 micron thick, thereby considerably improving the sensitivity of the sensor.
One of the drawbacks of the above teachings is that the design rules requiring relatively large chamber dimensions are a direct consequence of the low sensitivity of the device to applied pressure. As a result, when the pressure sensors are arranged in a two dimensional array to measure pressure distributions individually at various points, the lateral two dimensional resolution is on the order of 800 microns to 1 mm. This makes the devices unusable if high resolution is required. Reduction of these dimensions to miniaturize the structure will result in reduced sensitivity, fracture of the membrane in response to high pressures, and a starting low nominal capacitance value which increases the noise in the measured signal and therefore makes the measurement less reliable.
A second type of semiconductor based pressure sensors uses the same type of sealed chamber described above, but detects deflection changes using piezoresistive strain elements. The sealed chamber is made on a silicon wafer using similar methods to those described above. The membrane is made with a thickness in the range of 0.1–1 micron, and with a length in the range of 50 micron. Piezoresistive strain gauges are made of p-type silicon that are formed on the membrane. The deflection of the membrane under applied pressure causes changes in the resistances of the strain elements, which can be used to determine pressure.
However, there are several disadvantages with this approach. First, the sensitivity of this device is very low, with a gauge factor, defined by the term (ΔR/R)/ε (where ΔR/R is the relative change in resistance, and ε is the strain in the material) of about 120. Second, the thickness of the diaphragm, at 0.1–0.3 micron, is so small that excess pressures will tend to fracture the beam. Third, since silicon is the only piezoresistive material whose properties are well known, characterized, and lends itself to manufacturing, any pressure sensor based on piezoresistance is restricted to the use of silicon substrates. Finally, when the sensors are made in a two dimensional array to measure pressure distributions, the resolution is on the order of 200 microns.
Other patents related to the art of providing semiconductor pressure sensors, but having similar drawbacks to those identified above, include U.S. Pat. Nos. 4,771,638, 4,498,229, 5,427,975, 4,771,639, 5,736,430, 4,809,552, 5,471,723 and 4,744,863.
Use of Pressure to Image Fingerprints
The fingerprint sensing industry uses several different technologies to image fingerprints. The two most prominent technologies are optical based sensors and capacitance based sensors. Optical sensors use a light source, lenses and a prism to image the “ridges” and valleys on a fingerprint, based on differences in the reflected light from the features. The capacitance sensor uses semiconductor type processing to fabricate a two-dimensional array of capacitors. The individual sensors form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate. Upon contact with the array of sensors, the individual distance from each sensor to the skin is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint provide the means to replicate the fingerprint.
Both the above techniques fundamentally measure the spacing between the fingerprint features, and the sensor. The difference in spacing provides the means to differentiate between the high points (valleys) and the low points (ridges). The measurement of spacing is inherently subject to several drawbacks: since the height difference between the ridges and valleys is only of the order of 50 microns, any parameter which affects the spacing between the finger and the sensor will affect the measurement. For example, both types of sensors are very sensitive to the thickness of the protective coating. They are also sensitive to oils or grease on the finger, and the presence or absence of moisture on the finger. In addition, most of these sensors are affected by the ambient temperature at the time of sensing. Under very hot or very cold conditions, capacitive sensor can provide erroneous readings.
As a result of the above drawbacks to spacing based reproduction of fingerprints, it would be very useful to be able to use the difference in pressure exerted by the ridges and valleys of a fingerprint on a sensor to replicate the fingerprint image. In principle, a pressure based fingerprint sensor would be impervious to the drawbacks listed above, such as wet or dry conditions on the fingertip, presence of oil or grease on the fingertip, thickness of protective coatings, etc. However, due to the very low sensitivity and inability of the prior art to provide the required resolution, pressure based sensors have not been deployed for the replication of fingerprints.
Accordingly, there remains a need in the art for a microelectromechanical device suitable for use as a stress and/or pressure sensor that has high sensitivity yet can provide high lateral resolution. Moreover, there further remains a need for a sensor that is suitable for use in fingerprint identification and verification that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease.